In recent years, interpolymer complexes have received considerable attention in the literature due to their interesting and unique properties. In most instances, these complexes are formed by intimately mixing aqueous solutions containing high-charge density polyelectrolytes possessing opposite charge. When these polymer molecules meet in solution, the interaction between oppositely charged sites will cause the release of their associated counterions forming the complex. The counterions are now free to diffuse into the bulk solution. Normally, phase separation occurs upon prolonged standing in these high-charged density complexes. As a result, these materials have poor rheological properties. In recent work we have found that low-charge interpolymer complexes are soluble and effective in viscosifying aqueous solution systems. More importantly, these complexes possess a substantially higher viscosity than the corresponding individual low-charge density copolymer components. These characteristics are unexpected since high-charge density complexes are insoluble in these conventional solution systems. Therefore, it is anticipated that few detailed rheological studies of these latter materials appear in the literature. In particular, shear rate measurements are markedly absent.
Polymeric materials are useful as viscosity enhancers when dissolved in the appropriate solvent system. The principle reason for this behavior is due primarily to the large volume which a single macromolecular chain can occupy within the solvent. An increase in the size of the chain produces a concomitant enhancement in the solution viscosity. However, when the polymer chain is placed in a shear field, segmental orientation takes place in the direction of the shearing force. The viscosity of the fluid dramatically drops due to this orientation phenomena. Such shear thinning behavior is typical of most solutions containing dissolved polymeric materials. However, if the polymer molecular has a very high molecular weight with a relatively flexible backbone and the solvent viscosity is sufficiently high, different behavior can be anticipated. It has been shown by several groups that, with increasing shear rates, the viscosity should show a decrease, followed by a minimum value and a subsequent small increase in cases where both solvent viscosity and polymer molecular weight are very high. This latter effect gives rise to a very mild dilatant behavior. However, the above-mentioned conditions required for the appearance of shear thickening behavior in these polymeric solution systems are not applicable for many technologically interesting fluids. In most of the common synthetic polymers, it is difficult from a synthetic viewpoint to obtain sufficiently high molecular weight and even when obtained it is easily degraded under shear, in addition, most solvents (for example, water) have rather low viscosities.
This invention discloses the novel and unexpected result that soluble interpolymer complexes of lower molecular weight are capable of enhancing the viscosity of aqueous solutions under relatively broad shear conditions. With these unique polymeric materials, dilatant behavior occurs in aqueous fluids which are of extreme technological utility. It is further observed that under the identical experimental conditions, the viscosity of the individual copolymer components show the normal shear thinning behavior.
Polymers with very high molecular weight can be used to modify a solvent for a variety of technological applications. In this invention it is disclosed that an alternative to ultra high molecular weight additives are lower molecular weight polymers which are capable of associating in solution, thereby building a network of a very high molecular weight. A way for achieving such networks is the complexation of two dissolved polymers, one having anionic charges along its backbone and the other having cationic charges along its backbone. The complex can be achieved by dissolving each polymer alone in the solvent and combining the two solutions. Alternatively, each polymer can be codissolved in the same solution system. When polymer molecules of opposite charges meet in solution, an interaction occurs between oppositely charged sites forming a complex which involves the associated counterions that may have been present in one or both polymers.
In order to avoid phase separation of the complex in solution, the charge density along the polymer backbones should be relatively low. The resulting solution of such a complex is then significantly more viscous than solutions containing the individual polymers, provided that the total numbers of negative and positive charges are correctly balanced. Upon addition of a strongly polar agent such as water soluble inorganic salts alcohol the complex can be disturbed and the viscosity reduced.
It was found that for a given balance of the various parameters that may be varied in an interpolymer complex solution, an unexpected shear thickening (dilatant) behavior may be obtained. These parameters include:
Backbone nature of each of the polymers (or copolymers). PA1 The charge densities along the polymer backbones. PA1 The molecular weight of each polymer. PA1 The ratio between the polymers introduced into solution. PA1 The solvent (and cosolvent, if any). PA1 The concentration of polymer in solution.
As explained above, most solutions of high molecular weight polymers are expected to exhibit a shear thinning behavior. Interpolymer complexes under narrow conditions seem on the other hand to possess an ability to establish even larger networks or act as if networks are larger under high shear rates resulting in shear thickening.
For example, shear thickening behavior can be useful in affecting antimisting characteristics. Such a solution can behave as a fairly low viscosity fluid at low shear rates. However, the viscosity begins to rise as the shear rate is progressively increased. Accordingly, the solution can more effectively resist breakup into a mist of minute droplets. This is a very desirable attribute in a variety of fluids of technological interest. Another desirable attribute is to be able to reverse (or erase) the above-mentioned antimisting behavior and render it atomizable. With regard to interpolymer complexes, this is readily achieved through addition of a soluble component capable of weakening or totally disrupting the ionic linkages which hold the complex together. Such a component should be highly polar, soluble in the solution containing the dissolved interpolymer complex and capable of efficiently migrating (and disrupting) to the ionic linkages. Water soluble salts, acids, bases and amines are some possible examples.